gnubg: A Backgammon implementation - to lay and analys games and matches1

It is currently a work-in-progress. So far it is able to play both
independent games and tournament matches, evaluate and roll out
positions, tune its own evaluation functions using either TD or
supervised training, maintain databases of positions for training
and other purposes, and more.

It is driven by a command-line interface, and displays an ASCII
rendition of a board on text-only terminals, but also allows the
user to play games and manipulate positions with a GTK+ board window
where available. It is extensible on platforms which support Guile.

It currently plays at about the level of a championship flight
tournament player (depending on its parameters and its luck in recent
games, it rates at around 2000 on FIBS, the First Internet Backgammon
Server -- at its best, it is in the top 5 of over 6000 rated players
there) and is gradually improving; it should be somewhat stronger than
this when released. Since almost all of the CPU time required during
supervised training is spent performing rollouts, and rollouts can
easily be performed in parallel, it is hoped that users will be able
to pool rollout results and collectively train it to a level stronger
than any individual could obtain.